North Korea has test-fired five short-range missiles into the sea off its east coast, raising cross-border tensions ahead of Seoul's planned joint army drills with the US, according to South Korea.

The North fired the missiles into the Sea of Japan from its eastern city of Wonsan between 4:20pm-5:10pm local time on Sunday (07:20-08:10 GMT), the country's defence ministry spokesman told the AFP news agency.

"We are closely watching for any signs of additional missile launches by the North," said the spokesman, addign the missiles flew about 200km before landing in the water.

On Saturday, the North said it had test-fired an "ultra-precision," anti-ship rocket, which will be deployed across its navy "before long".

The missile tests come as South Korea and the US prepare to start major annual joint military exercises early next month.

The large-scale 'Key Resolve' and 'Foal Eagle' exercises look set to trigger a sharp rise in military tensions on the divided peninsula.

Seoul and Washington also started last week a joint naval exercise involving a US nuclear submarine.

Pyongyang sees all joint US-South Korea army exercises as provocative rehearsals for invasion and has frequently demanded they be stopped.

Nuclear tests

North Korea has pushed ahead with the development of ballistic missiles and rockets, despite tough sanctions imposed by the international community.

In 2012, Pyongyang demonstrated its rocket capabilities by sending a satellite into orbit, but it has yet to conduct a test that proves it has mastered the technology required for an effective intercontinental ballistic missile (ICBM) that could reach the US mainland.

Seoul's military said in January that the North had also made "significant" steps in developing technology that would allow it to mount nuclear bombs on a missile.

Pyongyang has staged three nuclear tests, most recently in February 2013 - the first since young ruler Kim Jong-un came to power and its most powerful to date.

The Korean Peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty.

Source: Agencies